In general, liquefied natural gas (LNG) refers to a colorless, transparent, and ultra-low temperature liquid obtained by cooling methane-based natural gas at about −163° C. and reducing the volume thereof to 1/600.
The use of LNG as an energy resource has brought about the need for an efficient transportation means capable of carrying large amounts of LNG from a production base to a supply depot to use it as energy. LNG carriers capable of carrying large amounts of LNG by sea have been developed as part of this effort.
By the way, an LNG carrier has to be equipped with a LNG storage tank capable of containing cryogenically liquefied natural gas. The requirements for such an LNG storage tank are difficult to meet, causing many difficulties.
That is, since LNG has a vapor pressure that is higher than the atmospheric pressure and a boiling point of about 163° C., the LNG storage tank that stores LNG needs to be constructed with materials that can withstand very low temperature, for example, aluminum steel, stainless steel and 35% nickel steel, and designed in a unique insulation structure that can withstand thermal stress and thermal contraction and can be protected from heat leakage, in order to keep and store LNG safely.
A conventional LNG storage tank is constructed of a rectangular lower insulation board, a secondary barrier, and an upper insulation board sequentially laminated on the inner hull of an LNG carrier, with a primary barrier being attached on the upper insulation board.
Particularly, the secondary barrier includes a main secondary barrier located between the lower insulation board and the upper insulation board and an auxiliary secondary barrier that entirely covers neighboring two main secondary barriers, with a thermosetting adhesive layer being interposed between the main secondary barrier and the auxiliary secondary barrier to attach the auxiliary secondary barrier to the main secondary barrier.
However, when carrying out pressing and heating processes using an external heat source in order to apply heat to the thermosetting adhesive layer, it is difficult to uniformly heat a wide area and maintain pressure for long periods of time.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.